On Wed, 2020-09-23 at 16:03 +0200, Christian König wrote: > Am 23.09.20 um 11:24 schrieb Hellstrom, Thomas: > > On Wed, 2020-09-23 at 13:17 +1000, Dave Airlie wrote: > > > On Fri, 18 Sep 2020 at 00:49, Christian König < > > > christian.koenig@xxxxxxx> wrote: > > > > Am 17.09.20 um 16:44 schrieb Michel Dänzer: > > > > > On 2020-09-17 2:20 p.m., Christian König wrote: > > > > > > Hi guys, > > > > > > > > > > > > Michel once submitted a patch to fix triggering this BUG_ON > > > > > > in > > > > > > ttm_tt_swapout(): > > > > > > > > > > > > > BUG_ON(ttm->caching_state != tt_cached); > > > > > > Now my question is does anybody know why we have that in > > > > > > the > > > > > > first > > > > > > place? > > > > > > > > > > > > The only problematic thing I can see is calling > > > > > > copy_highpage() > > > > > > and > > > > > > that one is just doing a kmap_atomic()/kunmap_atomic() on > > > > > > the > > > > > > source > > > > > > and destination. > > > > > > > > > > > > I can't see why it should be problematic for this temporary > > > > > > mapping > > > > > > to be cached instead of uncached or WC? > > > > > > > > > > > > Does anybody has any idea? > > > > > One thing is that AFAIK some (ARM?) CPUs can get very upset > > > > > when > > > > > there's both a cached and uncacheable mapping for the same > > > > > physical page. > > > > Good point, but I already considered this. > > > > > > > > If there is another mapping for that page left we are > > > > completely > > > > busted > > > > anyway since we are currently changing the underlying storage. > > > > > > > It's not just ARM of course, x86 PAT also has many issues about > > > multiple mappings of the same backing page with different > > > attributes. > > > > > > The only mappings might be in the linear mapping, since not all > > > pages > > > we get here will necessarily be high pages I assume and therefore > > > could have a linear mapping. If we've changed the linear mapping > > > to > > > non-cached, then create a cached kmap I'm not 100% that won't > > > cause > > > issues. > > > > > > but this is a definite maze of twisty passages and I'd probably > > > need > > > to sit down and break it a bit more. > > > > > > Dave. > > This is a problem that goes back way far (12+) years that the x86 > > architecture is not allowed to do aliased mappings of pages, even > > through mappable GTTs: There are two aspects: > > > > 1) Create a WC mapping of a page with data in the cache. When the > > cache > > does a writeback, anything written through the WC mapping will get > > overwritten. > > > > 2) Flushing the WB mappings first might not help, since at that > > time > > there were some AMD processors (Athlons?) that were speculatively > > prefetching data on the WB mapping into the cache at any time, and > > even > > though it wasn't changed it did a writeback. Anything written > > through > > WC in-between the prefetch and the writeback got overwritten. That > > was > > a real and occuring problem at that time. AMD claimed it was not > > violating specs. > > > > So aliased mappings is probably at best very fragile. > > All of this is correct, but I still can't see why ttm_bo_swapout() > tries > to change the caching? > > See copy_highpage() will only create a temporary wb mapping for > highmem > pages which is destroyed immediately again. Otherwise it will use > the > linear mapping which should already have the correct caching > attributes. > IIRC there are a couple of historical reasons - Generally avoiding aliased mappings with conflicting caching - Reading out from WC is dead slow - Not sure of the order of events, but I think swapping predates the WC page pool, which meant the pages had to change caching anyway. - A desire to be able insert the pages directly in the swap cache rather than to copy shmem objects. But today, we can probably do better. I guess, where available, streaming WC-tailored memcpy and not changing the caching might be a good choice? /Thomas ---------------------------------------------------------------------- Intel Sweden AB Registered Office: Isafjordsgatan 30B, 164 40 Kista, Stockholm, Sweden Registration Number: 556189-6027 This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
